How are things stored on CDs

How does a CD player read a CD-ROM?

Whether music, videos or computer programs are stored on a CD-ROM - a laser beam elicits its data from the colorful, shimmering disks; the phenomenon of interference helps.

CDs and DVDs have long been part of everyday life in computers and entertainment. In contrast to the old vinyl records, where the grooves were scanned with a needle, the data on CDs is read out contact-free with a laser beam. The inventors of the first types of silver disks, the read-only CD-ROMs, took advantage of the phenomenon of interference.


Interference always occurs where waves are involved and overlap. If, for example, two waves with the same wavelength collide, two extreme cases can occur:

  1. The wave crests of one wave are superimposed with the wave crests of the other, which results in wave crests that are twice as high: \ (1 + 1 = 2 \).
  2. Or: The wave crests of one wave collide with the wave troughs of the other, which results in a complete extinction of both waves: \ (1 + (-1) = 0 \).

In between there are an infinite number of other possibilities from 0 to 2. Which case occurs depends on how the two waves are shifted against each other.

Interference with the CD player

Reading out a CD

In order to read data from a CD, use is now made of the fact that displacements can be canceled out when laser light falls on it. A CD-ROM consists of a reflective aluminum layer with small recesses. The pits are only a quarter to a sixth of the wavelength of the light with which the CD-ROM is read. In the case of a conventional CD-ROM, red laser light with a wavelength of 780 billionths of a meter (nanometer) is used.

If the laser beam now completely falls on a depression or a non-depression, the beam is maximally reflected at a certain read-out angle. It looks different when the laser beam shines on the edge of a depression. Then part of it is reflected on the elevation and another part is reflected on the depression. Due to the height of the depression, a partially canceling interference occurs, which reduces the intensity of the reflected light.

This decrease in intensity can be measured with a light sensor. In this way, a CD player can register the edges on the surface (however, it cannot distinguish between normal surface and recess). If the edges are now interpreted as "0", for example, and the rest as "1", then these values ​​can be converted into music, videos or other data afterwards.

In addition to CD-ROMs, other silver disks are also used: on the write-once CD-R, for example, a burning laser can activate dyes and thus change the reflective behavior of the surface. The information is then read out via it.

Interference in Current Research

Interference phenomena play an important role in many measurement methods in physics - for example, when in astronomy the signals from several telescopes are interconnected to allow a clearer view of space, or when laser light is superimposed after a kilometer-long run to look for gravitational waves. But even when examining materials or biomolecules with intensive synchrotron radiation, one cannot avoid the phenomenon of interference. In this case, the researchers must use the interference patterns that arise when the specimen is exposed to infer its interior.